The resistance of human immunodeficiency virus type 1 (HIV-1) to zidovudine and the vertical transmission of the virus were assessed among all 62 HIV-1-infected pregnant women identified prior to delivery at one institution. HIV-1 was transmitted to infants from 11 (26%) of 42 women who did not receive oral zidovudine but from only 1 of 20 women given such treatment (P = .04). Isolates of HIV-1 from 16 of the 20 zidovudine-treated women were available. Twelve of 16 isolates were wild-type for pol codons 41, 67, 70, 215, and 219; two (one susceptible and one moderately resistant to zidovudine) had mutations at codon 70; and two (both highly resistant to zidovudine) had mutations at codons 41 and 215. The virus was vertically transmitted from a woman infected with one of the highly resistant strains, and the infant's isolate was highly resistant to zidovudine. These limited data suggest that maternal treatment with oral zidovudine reduces the rate of vertical transmission of HIV-1 but that vertical transmission of virus resistant to zidovudine can occur.